Yogurt is milk fermented with the aid of bacterial cultures. Greek yogurt is also known as “strained” because whey is filtered out, resulting in a thicker, creamier product. It takes a cup of milk to produce a cup of regular yogurt, but it takes three or more cups of milk to make a cup of Greek. That results in a lot of leftover whey—or “acid whey,” as it is known in the industry, because it’s acidic.

The whey can’t simply be dumped in a river “for the same reason that apple peeling can’t be dumped in a river,” says Andrew Novakovic, professor of agricultural economics at Cornell University. “It’s not that apple peelings are going to kill you, but natural systems like a river can only handle so much foreign biological material.”

The whey isn’t necessarily poisonous, although the acidity might prove a risk to fish and plant life in the immediate area of disposal. The bigger issue is that anything dumped into, say, a river would force the river to somehow digest it, which in turn depletes oxygen in the water, posing risks to fish and other wildlife.

Yogurt makers such as Chobani of Norwich, N.Y., the leading producer of Greek yogurt in the U.S., sell the leftover whey to farmers who use it mostly for fertilizer, spokeswoman Lindsay Kos says. Fage U.S.A., a Greek yogurt maker in Johnstown, N.Y., feeds its whey to the city’s wastewater treatment plant, where anaerobic microorganisms convert the waste to a renewable gas that can be used as an energy source.

Scientists at Cornell and elsewhere are exploring how protein in whey could be developed into food ingredients. Infant formula is one potential use, because yogurt whey is relatively high in the type of protein more common in human milk than in cow’s milk, Novakovic says.